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The R.4 was a hard vacuum valve developed from the R Type to meet a specific need for a heterodyne receiver. See the article above for full details. See also R2 and R5.

The lettering on the envelope reads: Valve wireless receiving. The other side reads: Osram GEC Made in England. R.4. The lettering reads top to bottom when the envelope is screwed into the screw holder. The R Type standard of the pinch above the base became dominant.

Thanks to Joe Gruber for sending the images.

The balloon envelope is 50 mm in diameter, and including the candelabra cap is 86 mm tall.

References: ¹⁰⁰³ & ¹⁰⁰⁴. Type R.4 was first introduced in 1917. See also 1917 adverts.